The present invention relates generally to a magnetic disk unit using a disk type recording medium (which will be hereinafter referred to as a "magnetic disk") such as a hard disk. More specifically, the invention relates to a magnetic disk unit which can greatly improve the positioning accuracy and speed between a magnetic head and an information recording track by recording positioning signals over the whole periphery of the magnetic disk.
In particular, the present invention relates to a small magnetic disk unit wherein a servo surface is the same as a data surface. In addition, the invention is applied to a magnetic disk unit having a multi-head system which can read information by means of a plurality of magnetic heads in accordance with the increased density of a magnetic disk.
In recent years, with the improvement of the performance of information apparatus such as a computer, it has been required to increase the recording density of a magnetic disk and the transfer rate for reading by means of a magnetic head in a magnetic disk unit. In order to satisfy such a request, the performances of recording disks and recording and/or reproducing heads have been energetically improved, so that the recording capacity of a disk has been greatly increased.
In conventional magnetic disk units, a single recording and/or reproducing head is arranged so as to face an information recording surface of a magnetic recording medium. This recording and/or reproducing head gains access to a desired track of the information recording surface of the magnetic recording medium by rotating a supporting arm having the recording and/or reproducing head at the tip thereof, so that servo signals recorded at several positions per track are reproduced by means of the recording and/or reproducing head. Thus, the information signal is reproduced while carrying out the fine positioning.
Although the recording capacities of magnetic disks mounted in magnetic disk units have been greatly increased, such constructions and operations have not basically been improved, so that the positioning between the magnetic head and the magnetic disk is carried out in the same manner as the conventional manner.
In a magnetic disk unit having a high-density magnetic disk, with the increase of the density of tracks formed on the information recording surface, the number of tracks increases and the width of the information recording track decreases. When the width of the information recording track decreases, there are problems in that it is required to accurately carry out the positioning of the magnetic head and track, and that the access speed of the magnetic head decreases due to the increased number of tracks.
In addition, in conventional magnetic disk units, information is recorded on a plurality of tracks provided on concentric circles, and read out of the tracks, by means of a magnetic head which is positioned at a radial position on a magnetic disk by rotating the magnetic disk. In order to position the magnetic head, the sector servo method, wherein servo information is discretely written on a part of the recording circle of a track, is utilized in a small disk such as 3.5-inch and 2.5-inch disks for a hard disk drive (HDD). This servo information comprises various informations. In the sector servo method, there is provided a discrete positioning servo system for deriving a positioning error of a magnetic head from the informations to control the magnetic head so that the positioning error is zero.
FIG. 1 shows an example of a position information of the servo information written on a disk. In FIG. 1, one track comprises a plurality of servo sectors. The servo information is written on the front portion of each of the servo sectors, so that it is possible to obtain a positional error signal on the basis of the servo information. Next to the servo information, a data information is written on each track. A transducer having a narrower width than that of the track is mounted on a magnetic head. The positioning of the transducer is carried out by causing a positional error information to feedback to a servo system every track. Four burst servo patterns of A, B, C and D are written as the servo information,. The tracks are expressed by six tracks of (n-2), (n-1), n, (n+1), (n+2) and (n+3). The transducer mounted on the magnetic head is positioned at track n.
Out of the four servo patterns of A, B, C and D, the positions of the servo patterns A and B (or C and D) are shifted from each other by one track, and the positions of the servo patterns A and C (or B and D) are shifted from each other by a half track. By deriving the difference between the positional signals obtained by the servo patterns A and B, it is possible to obtain a positional signal having a good linearity in the range of from -1/2 to +1/2. The servo patterns C and D are used for obtaining the positional informations of tracks between -1 and -1/2 and between +1/2 and +1.
In conventional magnetic disk units, a magnetic head is positioned at each track, and a serial data is read and written by means of the magnetic head. For that reason, there is a problem in that it is difficult to increase the transfer rate of data. In addition, the servo information read out by the transducer is used. Therefore, when the density of track increases, the width of track decreases and the read signal for the servo information decreases. As a result, there is a problem in that the signal-to-noise ratio increases to lower the positioning accuracy.
In the aforementioned conventional magnetic disk units, there are problems in that it is difficult to gain the track access with sufficient accuracy and speed as the increased track density of the magnetic disk and the increased information recording capacity, and that it is not possible to sufficiently increase the transfer rate of data in an information processing apparatus or the like using this magnetic disk unit due to the limits of recording and/or reproducing speed.
In a hard disk drive as an example of conventional magnetic disk units, a plurality of magnetic disks, each of which has a magnetic layer on a non-magnetic substrate, are laminated around a single rotational axis. A recording and/or reproducing head is mounted on an arm to face each of the magnetic disks. The arm is moved by means of an actuator to carry out the positioning of the head. When the hard disk drive carries out the recording and/or reproducing of information, the head does not directly contact the surface of the magnetic disk which rotates at a high speed, but it is designed to gain access a desired position on the surface of the magnetic disk while being slightly upwards apart from the surface of the magnetic disk. Thus, the head is designed to record or reproduce signals on tracks of concentric circles on the surface of the magnetic disk.
In order to increase the storage capacity in the aforementioned hard disk drive, an attempt to improve the linear density of a disk, i.e., the packing density in a longitudinal direction of a track, and an attempt to improve the packing density by decreasing the width of a track to increase the track density, have been made. In recent years, an active head, such as a MR head, using the magneto-resistance effect of a high read sensitivity has been actively developed, so that a great read signal can be obtained from a narrow track. The MR head is designed to convert a magnetic flux from a recording medium into an electric signal, using the characteristic that the electric resistance of a soft magnetic substance such as permalloy is varied by an external magnetic field. Since the read sensitivity of the MR head is proportional to a sense current flowing through the soft magnetic substance for converting a resistance variation into a voltage variation, it is possible to obtain a great output even if the relative speed of the head to the medium is small.
On the other hand, even if the MR head of a high sensitivity is used, the distribution of read sensitivities in a lateral direction of track has the shape of, not rectangle, triangle or trapezoid which has dull ends at the skirt portions thereof. Therefore, there is a problem in that a signal is reproduced from the adjacent recording track due to the skirt portions of sensitivity, i.e., the cross talk occurs.